P0304 on 2013-2016 Scion FR-S: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2013-2016 Scion FR-S

The Scion FR-S shares its platform and Subaru-built FA20 boxer engine with the Subaru BRZ. This means that issues, recalls, and fixes are often identical between the two models. While ignition components are the primary suspects for a P0304, early models (especially 2013) were subject to a significant valve spring recall (NHTSA Campaign ID: 18V720000) that could cause misfires and catastrophic engine failure. Therefore, it's critical to consider the vehicle's service history regarding this recall, as both the original defect and botched recall repairs have been documented to cause engine problems.

Symptoms You May Notice

• Rough or shaking idle
• Hesitation or stumbling during acceleration
• Loss of engine power
• Flashing or solid Check Engine Light
• Smell of unburned fuel from the exhaust
• Difficulty starting or extended cranking time

Most Likely Causes

1. Failed Ignition Coil 🔴 High Probability → Shop Ignition Coil The original coil-on-plug ignition coils on 2013-2014 models are a well-documented failure point, with revised parts available for later model years that are more reliable. Forum users on ft86club.com frequently cite coil failure as the top cause for a single-cylinder misfire, often resolving the issue completely.
   How to confirm: Swap the ignition coil from cylinder 4 with the coil from cylinder 2 (front, driver's side). Clear the codes and drive. If the code changes to P0302, the ignition coil is faulty. This is the standard, free diagnostic step recommended by the community.
   Typical fix: Replace the faulty ignition coil. It is often recommended to replace all four if they are original and high-mileage, preferably with the revised OEM part (from a 2017+ model year) to prevent future failures.
   Est. part cost: $40-$90 for aftermarket, $90-$125 for OEM
2. Worn or Fouled Spark Plug 🔴 High Probability → Shop Spark Plug Spark plugs are a standard maintenance item with a 60,000-mile replacement interval. An old, worn, or oil-fouled plug can easily cause a misfire. The FA20 engine uses Iridium plugs which are sensitive to wear and contamination.
   How to confirm: After confirming the ignition coil is good, remove the spark plug from cylinder 4. Inspect it for wear (worn electrode), carbon fouling (black and sooty), or oil fouling. The gap should be .024" (0.6mm). A significantly larger gap indicates a worn plug.
   Typical fix: Replace the spark plug in cylinder 4. It is best practice to replace all four spark plugs at the same time if they are near their service interval. Torque new plugs to 10.8-14.5 lb-ft in the aluminum heads.
   Est. part cost: $15-$25 per plug
3. Clogged or Failed Fuel Injector 🟡 Medium Probability → Shop Fuel Injector The FA20 engine uses both port and direct injection (D-4S). While generally reliable, failures of the direct injectors have been reported by owners on forums, sometimes leading to misfires. One owner on ft86club.com reported a P0304 that was ultimately diagnosed as a failed direct injector after spark and coil were ruled out.
   How to confirm: This is more difficult for a DIYer. A professional can perform a fuel injector balance test. A more involved DIY method is to swap the injector with another cylinder, but this requires disassembling the fuel rail and special tools for the direct injector seals.
   Typical fix: Replace the faulty fuel injector. One owner on ft86club.com reported a dealer quote of around $1,450 for a direct injector replacement. Another source estimates parts and labor between $1,385 and $1,611.
   Est. part cost: $170-$300 for the part

Rare But Worth Checking

• Broken Valve Spring (2013 models): This was a major issue on 2013 models, leading to a recall (Subaru Recall WTV-81, Toyota Recall JSC / NHTSA 18V-772). A broken spring can cause a misfire and severe engine damage. If the vehicle is a 2013 model, check if the recall work was performed.
• Botched Valve Spring Recall Repair (2013 models): The recall repair itself became controversial, with numerous owners reporting engine failures shortly after the service was performed. This was often attributed to improper application of RTV sealant by technicians, which could clog oil passages and lead to oil starvation and engine failure. A class-action lawsuit was filed regarding this issue.
• Camshaft Position Sensor Clearance: → Shop Engine Camshaft Position Sensor Toyota issued Technical Service Bulletins (TSB S-SB-0013-14 and S-SB-0033-13 Rev) for 2013 models indicating that misfire codes, including P0304, could be caused by the camshaft position sensor clearance being out of specification, also causing rough idle or long crank times.
• Vacuum Leak: A leak in the intake manifold gasket near cylinder 4 could lean out the air-fuel mixture and cause a misfire. This is less common for a single-cylinder misfire than ignition or fuel issues but should be considered.
• Low Engine Compression: Internal engine damage, such as to piston rings or valves, can cause a loss of compression and a persistent misfire. This is a worst-case scenario and should be investigated with a compression test after ruling out spark and fuel issues.

Diagnosis Steps

1. Read the trouble codes with an OBD-II scanner and confirm P0304 is present. Note any other codes, like P0300 or P0354.
2. Check freeze frame data to see the engine conditions (RPM, load, temperature) when the misfire occurred.
3. Locate cylinder 4 (driver's side, rear position) and cylinder 2 (driver's side, front position).
4. Swap the ignition coil from cylinder 4 with the coil from cylinder 2.
5. Clear the codes, then drive the vehicle to replicate the misfire conditions.
6. Re-scan for codes. If P0302 appears, the ignition coil is faulty and must be replaced.
7. If the code remains P0304, the coil is likely good. Proceed to inspect the spark plug in cylinder 4. Replace it if it is worn, fouled, or past its 60,000-mile service life. It's best to replace all four.
8. If the misfire persists, the next steps involve checking the fuel injector and its wiring. This may require professional assistance or advanced diagnostic tools.
9. If spark and fuel systems are confirmed to be working, perform a compression test to check for mechanical engine problems like bad rings or valves.
10. For 2013 models, check the vehicle's service history on Toyota's website to see if the valve spring recall (JSC/18V-772) was performed and when.

Parts You'll Likely Need

• Ignition Coil (OEM #SU003-04713 (replaces SU003-00417, SU003-04670)) — This is the most frequent cause of a single-cylinder misfire on the FA20 engine, especially in earlier models. The original part was superseded by a more reliable version.
  Trusted brands: Denso (OEM), NGK, Delphi
  OEM price range: $90-$125
  Aftermarket price range: $40-$90
• Spark Plug (OEM #SU003-04387 (NGK ILKAR8H6 / Denso ZXU22HPR8)) — Spark plugs are a routine maintenance item and a common cause of misfires if worn or fouled. The OEM plug is an Iridium type.
  Trusted brands: NGK (96024), Denso
  OEM price range: $20-$30
  Aftermarket price range: $15-$25

Related Codes That Often Appear With This One

• P0300 — If multiple cylinders are misfiring randomly, a P0300 (Random/Multiple Cylinder Misfire Detected) may accompany P0304, pointing to a more widespread issue like a vacuum leak or fuel delivery problem.
• P0354 — This code means "Ignition Coil 'D' (Cylinder 4) Primary/Secondary Circuit Malfunction," which points directly to an electrical problem with the ignition coil or its wiring, rather than a mechanical or fuel issue. A user on Reddit noted that P035x codes are a clearer indicator of a coil pack electrical failure.

Technical Service Bulletins (TSBs) & Recalls

• Subaru WTV-81 / Toyota JSC (NHTSA 18V-772): Engine Valve Spring Recall for 2013 models.
• Toyota S-TT

Platform-Specific Known Issues

• A recall was issued for 2013 model year Scion FR-S vehicles for engine valve springs that could fracture, causing misfires, engine stalling, or engine failure (NHTSA ID: 18V-772). It's crucial to verify if this recall has been performed on any 2013 model.
• The valve spring recall repair itself became controversial, with a significant number of owners on forums like ft86club.com and in news articles reporting engine failures shortly after the service was performed, allegedly due to improper application of sealant by technicians leading to oil starvation. This led to a class-action lawsuit against Toyota.

Mechanic-Grade Diagnostic Values

• High-Pressure Fuel System Pressure — expected: 400 - 700 psi at idle; up to 2,500 - 3,000 psi under load.. Failure: Significantly lower pressure under load indicates a failing High-Pressure Fuel Pump (HPFP) or a restriction.
• Low-Pressure Fuel System Pressure — expected: 50 - 60 psi at idle.. Failure: Pressure dropping significantly under load suggests a weak in-tank fuel pump.
• Ignition Coil Primary/Secondary Resistance (Aftermarket Example) — expected: Primary: ~0.70 ohms; Secondary: ~3.7K ohms.. Failure: Readings that are open (infinite resistance) or significantly deviated from spec indicate a failed coil winding.
• Fuel Injector Resistance (Port or Direct) — expected: ~12 ohms (High Impedance).. Failure: A reading far outside this value, or an open circuit, points to a failed injector coil.

Hidden / Shadow Codes Worth Checking

• Mode $06 Misfire Counts: The ECU tracks misfires on each cylinder even before they are frequent enough to trigger a P0304 code. This data can show intermittent misfires that don't yet set a Check Engine Light. (see via A professional scan tool like Toyota's Techstream or an advanced OBD-II scanner with Mode $06 capabilities can display the raw misfire count per cylinder for the current and previous driving cycles.)

Scan Tool Commands That Help

• Toyota Techstream: Misfire Count (Live Data) — Use this to monitor cylinder 4 in real-time during a test drive, especially when trying to replicate symptoms like a slight hesitation. The counter may show misfires occurring even if no code is pending. Note that counters may reset on key-off if the threshold for storing in history isn't met.
• Toyota Techstream: Active Test: Check the Cylinder Compression — This is a relative compression test performed via software. It can help identify a mechanical issue with cylinder 4 without using a manual gauge. Be aware of a known bug on some models where cranking must begin within 1 second of starting the test for it to work.
• Toyota Techstream: Active Test: Cylinder Cut / Injector Cut — This function (if available on the specific ECU calibration) allows the technician to disable the injector for cylinder 4 while the engine is idling. A minimal drop in RPM indicates cylinder 4 was not contributing power, confirming the misfire location.

Wiring & Ground Locations

• Engine Harness Grounds — Typically bolted to the top of the intake manifold and on the cylinder heads.. The ignition coils and fuel injectors rely on clean, secure grounds to function. A loose or corroded ground connection for the bank that includes cylinder 4 can cause a weak spark or erratic injector behavior, leading to a misfire.
• Cylinder 4 Ignition Coil Connector — On the driver's side (LHD) of the engine, the rearmost coil.. The plastic connector and its wiring can become brittle from heat cycles. Inspect for cracked locking tabs, pushed-out pins, or chafed wires right at the connector body, which can cause an intermittent connection and misfire.

Real Owner Repair Stories

• Reddit user in r/ft86 (2013 Scion FR-S) — Car was missing slightly at idle but not consistently throwing a code.
  ❌ Tried (didn't work) Initial diagnosis was difficult as the code was not always present.
  ✅ What actually fixed it The car finally threw a P0302 code. Replacing the ignition coil for that cylinder made the car run much better. The user noted the other original coils were likely weak and contributing to the rough idle, and that a valve cover leak had fouled the combustion chamber. This confirms that a coil can be failing intermittently before it consistently sets a code.

"I Checked Everything" — The Actual Cause

• While a vacuum leak is a possible cause, a standard smoke test of the intake manifold may not reveal a leaking direct injector seal. These seals are under the injector tip inside the cylinder head. A failure can lean out a single cylinder, causing a misfire, but is often misdiagnosed as a faulty injector itself.

OEM Part Supersession History

• 22433AA652 (and others) → SU003-04713 / 22433AA730 (for 2015+) — The original ignition coils on 2013-2014 models had a higher failure rate. The parts were updated for the 2015 model year for improved reliability.
  Heads up: While the newer coils are a recommended upgrade, they have a different electrical connector. An updated engine sub-harness was also part of the 2015+ update. Swapping to newer coils on a 2013-2014 car may require wiring modifications or adapter pigtails.

Model Year Variations Within This Range

• 2013-2014: These models used earlier, less reliable ignition coils and wiring harnesses. 2013 models were also subject to the valve spring recall and had a different initial ECU tune.
• 2015-2016: These models received an updated engine wiring harness and improved ignition coils from the factory, making them less prone to P0304 from coil failure. The suspension was also revised for 2015.
• Mid-2014+: Minor internal engine changes were made, including updated rocker arms, rocker pivots, and valve stem lengths, potentially to improve valvetrain stability.

Diagnostic Flowchart

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0300 — Random/Multiple Cylinder Misfire Detected. Often appears alongside P0304 during severe misfires, as noted in owner reports on ft86club.com.
• P0302 — Cylinder 2 Misfire Detected. This code will appear if a faulty ignition coil from cylinder 4 is swapped to cylinder 2 during the recommended diagnostic test.
• P0354 — Ignition Coil 'D' Primary/Secondary Circuit. This code indicates an electrical issue with the cylinder 4 ignition coil or its wiring harness, and is often checked alongside P0304.

Frequently Asked Questions